The invention relates to a high-pressure gas discharge lamp with two tungsten electrodes which are each held in an electrode chamber by means of a support and which have a diameter of below 500 xcexcm.
Similar high-pressure gas discharge lamps are disclosed, for example, in WO 98/37570 A1 and WO 98/37571 A1. In these lamps, two tungsten electrodes, each also surrounded by a tungsten wire, project into an electrode chamber, supported by substantially freely projecting supports. The supports, which consist of or comprise rhenium in these publications, are in contact with the wall of the electrode chamber at a location denoted xe2x80x9cbottom of the electrode chamberxe2x80x9d. The electrode chamber is also sealed in this location. This is achieved by means of a metal foil which is connected to a respective support and a current supply wire and onto which the material forming the electrode chamber is pinched.
Of a completely different type, however, are high-pressure gas discharge lamps in which the tungsten electrodes have a diameter of less than 500 xcexcm or in which the bulb length, i.e. the distance between the bottom points of a wall forming the electrode chamber is less than 15 mm. The electrode chamber seals and the electrode chamber bottoms in such high-pressure gas discharge lamps lie in different positions. The electrode chamber bottom is here removed by up to several millimeters from the seals on the metal foil.
It is an object of the present invention to achieve that high-pressure gas discharge lamps with two tungsten electrodes, which are each arranged at supports in an electrode chamber and have a diameter of below 500 xcexcm, can be operated with a sufficiently long useful life.
To achieve this object, the invention proposes a high-pressure gas discharge lamp as described above, wherein at least one of the tungsten electrodes is entirely accommodated in the electrode chamber.
It was found that the useful life of the electrode assemblies formed from the support and the tungsten electrode can be substantially improved when the tungsten electrode is entirely arranged in the electrode chamber. Although the support as well as the tungsten electrode is substantially freely arranged also between the seal and the electrode chamber bottom, because the actual sealing of the electrode chamber takes place only behind the contact point of the support facing away from the tungsten electrode, a tungsten electrode extending from the electrode chamber to behind the electrode chamber bottom is subject to an increased wear. This can be avoided by a free arrangement of the tungsten electrode in the electrode chamber.
If on the other hand the electrode chamber is in contact with at least one support at a contact spot, and not at an electrode chamber bottom, the possibility is given that a sufficient convection takes place around the tungsten electrode, which enables a long useful life of the lamp.
It is of particular advantage when the sum of the electrode interspacing and the interspacings between the electrode tips and the respective electrode chamber bottoms divided by the electrode interspacing has a value of between 1.6 and 3. Such an arrangement safeguards on the one hand an advantageous generation of a temperature gradient over the electrode assembly comprising the tungsten electrode and the support as well as a sufficient convection around this electrode assembly, whereby a long useful life can be guaranteed.
Preferably, at least one support comprises a rhenium sheath or is formed by a rhenium rod. It is advantageous here when the rhenium content of the sheath or rod does not fall below 25% by weight. Such an arrangement is particularly suitable for high-pressure gas discharge lamps which contain a gas filling substantially composed of metal oxyhalides during operation. The electrode chamber in this case may comprise, for example, a mixture of NaI, SnI2, NaBr, TlBr, HgI2, and/or WO3. In addition, mercury, oxygen, and argon or a different rare gas may be provided in the electrode chamber. The oxygen, or the WO3 as an oxygen provider, serves to form oxyhalides here.
Preferably, the tungsten electrode is welded on the support. Such a connection can be prepared in a reliable way also in very small dimensions. This is true in particular in relation to the use of rhenium.
It is particularly difficult, especially in the case of small lamp dimensions, to safeguard that the tungsten electrode is present in its entirety in the electrode chamber. This is true in particular with respect to an industrial manufacture and the tolerances inherent therein. It is suggested, therefore, that the electrode chamber in at least a cross-sectional plane has a tapering shape towards at least one electrode or towards at least one support, the taper gradient in the tapering direction passing through a real minimum up to a contact spot or an electrode chamber bottom. Such a construction of the electrode chamber can guarantee an artificial lengthening of the electrode chamber in the region of the electrode chamber bottom, which lengthening extends away from the tungsten electrode. It can be reliably achieved by means of such a lengthening that the tungsten electrode lies in its entirety in the electrode chamber.
This can be safeguarded in practice in that in the manufacture of the high-pressure gas discharge lamp, during which the lamp body is molded into shape and the gas-filled electrode chamber is formed with its electrodes projecting from the electrode chamber bottoms into the electrode chamber in the interior, this lamp body after its formation is subsequently widened in the region of at least one electrode chamber bottom. It is obvious in this case that also a fluent transition between the formation of the lamp body by molding and the subsequent widening may also be provided.
The lamp body may be heated in the region of the electrode chamber bottom for the purpose of widening. This safeguards a particularly simple manufacture of the respective lamp.
It is obvious that a taper gradient which passes through a real minimum and a relevant manufacturing process may also be advantageously utilized independently of the other characteristics of the high-pressure gas discharge lamp, in particular independently of the type of electrodes, so as to be able to manufacture sufficiently freely exposed electrodes also in small lamps, in particular also under mass manufacturing conditions. Lamps can be manufactured in this manner in particular whose bulb length is shorter than the inner-wall chamber, i.e. than the sum of the electrode interspacing and the distances between the electrode tips and the respective electrode chamber bottoms.